1. Field of the Invention
This invention relates to medical devices for the reception of radio frequency electromagnetic radiation. These devices are used to obtain a local but very wide field-of-view magnetic resonance image of a region within a natural organism (such as within a human) or elsewhere. Medical devices such as catheters and other devices for delivery of therapeutic agents and monitoring of metabolic activity may be used together with such magnetic resonance imaging devices. The use of novel microcoil configurations in the devices alter the response characteristics of the microcoils.
2. Background of the Art
Throughout this specification, the term MR is used to mean xe2x80x9cMagnetic Resonancexe2x80x9d and xe2x80x9cMR microcoilxe2x80x9d is used to denote a magnetic resonance device used for imaging from within a patient. MR coils are conventionally used externally to the body in order to generate MR images, while the MR microcoil may be mounted at the tip of a catheter or other insertion device used commonly to probe the interior of a body so as to provide quick and direct access to the region where imaging is required. It is essential during the course of medical procedures such as image-guided and minimal access surgery, performed within small regions of a patient""s anatomy, to be able to visualize the procedure being performed by the surgeon and the neighborhood of the anatomical region being treated surgically. While several methods, including x-ray imaging and fiber optic viewing offer possible alternative means of performing the visualization, magnetic resonance imaging methods are a particularly convenient means of doing this, especially given the highly localized nature of the procedures being performed. Extended x-ray exposures are harmful to the patient, and fiber optic viewing is not well suited either to viewing within small confines or to volume visualization. Both of these limitations may be circumvented by magnetic resonance imaging.
In addition, as described in U.S. patent applications Ser. Nos. 8/857,043 and 08/856,894 filed on May 15, 1997, the use of improved Magnetic Resonance Imaging (MRI) techniques and devices enables a real-time visualization of compositional changes in the molecular composition of small regions within patients. The compositional changes may be caused by delivery of drugs or active chemicals, or by the stimulation of local chemical production by tissues or organs in the patient. MRI can actually enable visualization of minute concentration changes within the body, particularly intracranial regions of the patient.
U.S. Pat. No. 5,271,400 describes a tracking system for the position and orientation of an invasive device within a patient. The device includes a receiver coil and an MR active sample. In response to radio frequency radiation generated by an MR scanner system, the sample absorbs and re-emits this radiation as decaying radio frequency signals. The receiver picks up magnetic resonance signals re-emitted by the sample, from an analysis of which the location (position and orientation) of the device in space may be reconstructed. The frequencies are proportional to the location of the coil along the applied field gradients, since the signals are received in the presence of these magnetic field gradients. The system is designed to enable location of the invasive device and enhanced imaging of a region around the invasive device is not a functionality intended for this device.
In xe2x80x98MR imaging of blood vessels with an intravascular coilxe2x80x99, J. Mag. Res. Imag., 1992, Vol. 2, pages 421-429, A. J. Martin, D. B. Plewes and R. M. Henkelman describe an opposed solenoid design for an intravascular MR microcoil. This paper describes microcoils made of a pair of helical windings arranged in opposed fashion at the tip of a catheter, shown to be suitable for magnetic resonance imaging purposes. The term xe2x80x9copposed coilxe2x80x9d means a coil in which the relative winding of two coil segments is opposite in sense, and the current flow in each opposed coil winds in opposite directions about the coil axis (relative to moving towards or away from the core or axis of the coil). That is, viewing the coils looking down an axis of the core around which the coils are disposed, one will be wrapped clockwise and the other will be wrapped counterclockwise, with a common lead between the two segments. The field-of-view of this coil is roughly cylindrical about the opposed solenoidal windings. The coil is essentially radio frequency insensitive beyond the longitudinal extent of the windings since the magnetic field in this design is squeezed out of the gap between the windings and is only significantly large in a cylindrical region that does not extend too far beyond this gap.
E. Atalar et al. describe a catheter receiver coil in xe2x80x98High resolution MRI and MRS by using a catheter receiver coilxe2x80x99,Mag. Res. Med., 1996, Vol. 36, pages 596-605. The gain of this coil falls off rapidly with distance from the coil so that noise levels in an image adjusted for the signal may still vary widely across the image. In addition, the gain (signal-to-noise ratio) provided by this design may be significantly improved upon by other means, such as in the invention described herein.
U.S. Pat. No. 5,271,400 describes a tracking system for the position and orientation of an invasive device within a patient. The device includes a receiver coil and an MR active sample. The receiver picks up magnetic resonance signals generated by the sample. The frequencies are proportional to the location of the coil along the applied field gradients, since the signals are received in the presence of these magnetic field gradients.
U.S. Pat. No. 4,572,198 describes a catheter for use with magnetic resonance imaging systems, the catheter including a wound coil for exciting a weak magnetic field at the catheter tip. This construction provides a local distortion of the MR image, yielding an image cursor on the magnetic resonance imaging display.
U.S. Pat. No. 5,964,705 describes an opposed solenoid design for an MR microcoil with helical winding""s whose pitch varies along the length of the winding with the aim of achieving homogeneity. However, the optimization method given there for finding a suitable pitch variation assumes a xe2x80x98sheet currentxe2x80x99distribution of the current along the catheter tip, which may not be not realized in practice. Accordingly, the homogeneity of the field produced by the microcoil can be improved by other means, such as the invention described and claimed herein.
A copending, commonly assigned application, filed the same day as this application and bearing Attorneys"" Docket No. 723.030US1 in the name of Raju Viswanathan, titled A Microcoil Device with a Forward Field-of-View for Large gain Magnetic Resonance Imagingxe2x80x9d describes a microcoil configuration wherein a device to be inserted into a patient comprises a solid body having at least one microcoil physically associated with the solid body, each microcoil having an outside microcoil diameter of 6 mm or less and a common axis, with at least one microcoil physically associated with the solid body at a distal end, at least 50% of individual windings of said each microcoil intersecting a geometric plane perpendicular to said common axis.
Copending U.S. patent application Ser. No. 09/532,145, filed the same day as this Application, and titled xe2x80x9cA DEVICE FOR HIGH GAIN AND UNIFORMLY LOCALIZED MAGNETIC RESONANCE IMAGINGxe2x80x9d discloses a microcoil configuration, preferably on a medical device to be inserted into a patient, that has an opposed pair of microcoils. At least one or each microcoil of the opposed pair of microcoils has at least a region where a diameter circumscribed by a first winding is greater than the diameter circumscribed by at least one complete adjacent second winding, especially an adjacent winding displaced from the first winding along an axis or core of the medical device or an axis of the microcoil. The second winding is nearer to or farther from an intermediate region between the microcoils that define the pair of microcoils. For example, it is common to have a connecting (usually straight or non-wound) lead between the two microcoils, and this lead may be used to define an intermediate region. The microcoil configuration with varying circumference between windings (especially adjacent windings) is generally referred to herein as a dumb-bell or horn configuration because of its general appearance and the individual microcoils are referred to as a horn microcoil, again because of the visual appearance of the microcoil.
In summary, while microcoils for such internal imaging have been described before, the said device here advances the art by virtue of being designed specifically to maximize the field of view as well as the signal gain within the field of view. This requires a tradeoff with homogeneity of the radial receptive field, which may however be corrected for if the receptive field (magnetic field B transverse to the main MR magnetic field produced by unit current in the coil) is known. This correction may be performed by dividing the reconstructed image intensity at a given pixel location by the gain corresponding to that pixel (which is proportional to the magnetic field B at that location), and repeating this for all pixels.
Microcoil designs are provided that enable unique RF response Field profiles that are particularly useful in MRI imaging procedures, particularly where fields of view outside of the medical device are desirable. These devices are particulatly for use within an organism, the device comprising an element having at least one RF receiver, the coils of said microcoils defining a cross-section that lies in a plane oriented at 0 to 90 (or 0 to 80) degrees to the longest axis of the device. Another way of describing the device is as a device for use in an organism, the device comprising an element having at least one wound microcoil with at least three windings on the microcoil. Each winding has an aspect ratio of greater than one. The aspect ratio of each winding is measured as the ratio of longest to shortest dimension in a cross section situated approximately transverse to the winding axis of the coil windings, the winding axis also being transverse to the longest axis of said device
Another way of describing the device for use within an organism is as a device comprising an element having at least one RF receiver microcoil, the coils of the microcoils defining a cross-sectional contour having an alignment value of at least 0.75 with the longest axis of the device. The device may have the cross-section comprise a geometric shape, such as a curvilinear shape, a polygon (regular or irregular), or a polygon where comers on the polygon are softened (e.g., slighly rounded). The device may comprise a catheter having at least one lumen. At least one microcoil should be located with its longest dimension defining a longitudinal spatial extent (direction) parallel to the at least one lumen and the coils having a conductor thickness of greater than 0.01 mm and less than 2.4 mm.